For example, in a digital radio communication device, in order to prevent adjacent-channel interference caused by higher-order distortion of a power amplifier, the linearity of the input and output amplitude characteristics of the amplifier is strictly required. However, it is difficult to achieve both high power and an increase in efficiency while maintaining excellent linearity.
Hence, to achieve an improvement in power efficiency while ensuring the linearity of the input-output characteristics of the power amplifier, there are proposed various distortion compensation schemes to compensate for distortion occurring in a nonlinear region. For such distortion compensation schemes, there are broadly a feedforward scheme, a feedback scheme, and a pre-distortion scheme.
Of them, the pre-distortion scheme adds a characteristic inverse to the distortion characteristic of an input signal to the power amplifier to the input signal in advance, and thereby obtains an output signal from the power amplifier in a “no distortion” or “very little distortion” state.
For a distortion compensation circuit of such a pre-distortion scheme, there are a scheme (LUT scheme) in which, for example, as shown in Non Patent Literature 1, the amount of correction is stored in an LUT (Look Up Table) and is sequentially modified using the difference between an output signal from the amplifier and a target output signal; and a scheme (polynomial approximation scheme) that performs adaptive control by approximating the amount of correction for the amplifier by a polynomial and computing the coefficient values thereof using an output signal and an input signal from/to the amplifier.